When handling syringes or dripsets for medical administering purposes, or during production or transport of such devices, there are a variety of possibilities wherein solid particles of, for example, dust can enter into the syringes or dripsets or into the liquids introduced into or stored in the syringes or dripsets. These particles may include not only dust or other fine particles which enter the syringes or dripsets from any external source, but also, fine fragments of glass or rubber which are produced when an ampoule or other breakable container of a liquid medicament is opened up or unsealed. Particles can also be formed during a process in which a rubber plug forming part of a dripset is fitted to a liquid reservoir or a liquid conducting tube of the dripset prior to practising a medical administering operation. When a medical solution or blood thus containing solid particles therein is introduced or transfused into a human or an animal body, the solid particles will find their way into the blood streams or body tissue and may injure the vascular tissue of the human or animal body.
Therefore, the provision of a filter medium in a syringe or a dripset for the purpose of separating fine solid particles from the liquid medicament or blood to be dispensed from the syringe or dripset has been proposed and practiced. The filter medium is usually located in the needle holder or the barrel of a syringe or can be located in the needle assembly of a dripset for collecting solid particles from the liquid medicament or the blood to be discharged from the syringe or the dripset through the cannula or injection needle attached to the syringe or forming part of the needle assembly of the dripset.
The filter medium thus incorporated into a syringe or a dripset is typically a fibrous gauze filter made of a woven fabric of, for example, nylon. A filter medium of this type is useful for collecting relatively large-sized solid particles such as, for example, the minute fragments of rubber which tend to be produced when a rubber plug having a liquid inlet needle inserted therethrough is forced into the neck portion of the plasic or glass bottle or vial of a dripset. The fibrous gauze filter is, however, not capable of collecting microscopically fine particles which may be deposited on the internal surfaces of the component elements of a syringe or a dripset. This allows such particles to pass through the filter medium into the vascular tissue of the patient to whom the administration using the syringe or dripset is to be conducted.
As is well known in the art, the sizes of the solid particles contained in or allowed in medical solutions or transfusion blood to be administered by syringes and dripsets usually range from the order of microns to the order of hundreds of microns in diameter. In order to collect solid particles having diameters of the order of microns at an acceptable efficiency by the use of a filter medium, the filter medium must be formed with pores, meshes or other voids measuring tenths of microns in diameter, length or width.
Attempts have therefore been made to provide microporous or micromesh filter media having pores, meshes or voids of such sizes. These attempts are largely directed at improving the quality of filter materials by making the filter materials denser having finer pores or meshes. For example, a micromesh filter of nylon, presently available on a commercial basis, is made up of densely interwoven nylon yarns of a minimum count or of warps and wefts which have different counts. Another example of the microporous or micromesh filter media which are in use today for medical administration purposes is a so-called membrane filter which consists of a disc or an otherwise shaped mat of sintered nylon powder.
While these known filter media are successful in collecting solid particles which are far smaller than before, problems are still encountered in that filter media tend to be clogged and cause deterioration of the filtering efficiency during use of the filter media. In certain situations, the filter medium per se becomes a source of impurities if the filter medium, such as a disc of sintered nylon powder, is partially fractured or fissured by the pressure of the liquids to be passed therethrough.
When, on the other hand, a filter medium is to be used for a medical administering dripset, it is preferred that the filter medium be provided in a flexible liquid conducting tube forming part of the dripset. Due to the limitation in the inside diameter of the tube, however, difficulties are experienced in mounting and positioning the filter medium securely and correctly in the tube. If the filter medium fails to be securely mounted or corrected positioned in the liquid conducting tube, the filter medium may lose its function or may at least be unable to exhibit its potential capabilities and preformance efficiency. If, furthermore, the filter medium provided in a liquid conducting tube of a medical administering dripset has pores or meshes having sizes so as to collect particles which are microscopically fine, the filter medium sized to be compatible with the fluid conducting tube will be easily clogged by the particles. This causes a reduction in the filtering efficiency thereof during use of the dripset. The reduction in the filtering efficiency of the filter medium results in a decrease in the flow rate of liquid through the liquid conducting tube in which the filter medium is provided. If the filtering efficiency of the filter medium used in a medical liquid dispensing device is reduced to a critical degree, the patient under administration may be exposed to a danger to life. In order to avoid such a danger, a compromise must be made by sacrificing the filtering efficiency for prevention of the clogging of the filter medium. A conventional microporous or micromesh filter medium for use in the liquid conducting tube of a medical administering dripset is, for this reason, actually capable of collecting only macroscopically sized particles out of the solid particles contained in the liquid to be passed through the tube.
The present invention is based on a discovery that all of these problems encountered in prior-art microporous or micromesh filter media for use in medical liquid dispensing devices can be eliminated by the use of a hollow microporous filament as a filter element.
As is well known in the art, a hollow microporous filament used mainly in the field of textiles is formed of polyvinylalcohol (PVA) or a rayon fiber and has inside and outside diameters of the order of hundreds of microns and pore sizes ranging from microns to tens of tenths of a micron. Such hollow microporous filaments have thus far been used not only as textile materials but as filter elements in some fields of chemical and biochemical industries for the separation of a polydisperse colloid into substances having different molecular dimensions. As a means for separating macromolecular and micromolecular substances from each other, for example, a hollow microporous filament is used for the demineralization and refining of an enzyme or the demineralization of hormone liquor in the field of biochemistry. When applied to the separation of micromolecular and micium-molecular substances from each other, a hollow microporous filament is utilized for the hypochlorization of soy sauce. In the separation, furthermore, of micromolecular substances from a mixed solution thereof, a hollow microporous filament is used for reclaiming an acid or alkali substance from the waste liquor produced in an electroplating process. In any of these known filter devices using hollow microporous filaments, however, such problems that are encountered in microporous filters for use in medical liquid dispensing devices as previously pointed out are usually not encountered. Even if the problems might be encountered, the problems would be far less serious than in the case of the medical liquid dispensing devices in which the dimensions of the spaces available for accommodating the filters are subject to an exacting limitation.
To use a hollow microporous filament of the above described nature as a filter element in a medical liquid dispensing device has been known per se from, for example, Japanese Provisionally Published Patent Specifications No. 53-48392 and No. 53-49885. In a medical liquid dispensing device shown in each of these provisionally published patent specifications, an elongated, hollow microporous filament which is open at one end and closed at the other is secured at the open end to a joint element having a passageway with which the passageway in the microporous filament communicates through the open end of the filament. The joint element is fitted to an end portion of a flexible liquid conducting tube of a medical administering dripset so that the hollow microporous filament axially projects from one end portion of the joint element into the passageway in the liquid conducting tube. The liquid medicament or the transfusion blood directed under pressure into the liquid conducting tube from a vertically inverted bottle held in a raised position is passed through the microscopic pores in the hollow microporous filament into the passageway in the microporous filament and through the passageway in the filament into the passageway in the joint element. The solid particles which may be contained in the liquid medicament or transfusion blood entering the hollow microporous filament are in this fashion collected on the outer peripheral surface of the filament projecting into the liquid conducting tube.
The hollow microporous filament thus arranged in a prior-art medical administering dripset serves not only as a filter element but as a microscopically perforated partition element between the passageway upstream of the microporous filament and the passageway downstream of the filament. In order for the hollow microporous filament thus serving as a partition element to play the part of a filter element in the liquid conducting tube, the passageway in the liquid conducting tube must be larger in cross sectional area than the hollow microporous filament so that the microporous filament has its particle-collecting outer peripheral surface exposed over the total area thereof to the liquid in the liquid conducting tube. If, in this instance, the hollow microporous filament is fitted to the joint element in such a manner that the filament has its open end portion received in an end portion of the passageway in the joint element, as is the case with the prior-art device under consideration, the passageway downstream of the hollow microporous filament becomes smaller in cross sectional area than the passageway upstream of the microporous filament. By reason of the difference thus existing between the cross sectional areas of the respective passageways upstream and downstream of the hollow microporous filament, the flow rate of liquid through the passageway downstream of the microporous filament becomes higher than the flow rate of liquid through the passageway upstream of the filament. When a liquid medicament or transfusion blood is passed through the hollow microporous filament, therefore, there is created a differential pressure between the respective passageways upstream and downstream of the microporous filament. The differential pressure acts through the microscopic pores in the hollow microporous filament on the solid particles which have collected on the outer peripheral surfaces of the microporous filament and, thus, urge the particles to move toward the passageway in the filament. As a consequence, those solid particles on the outer peripheral surface of the hollow microporous filament which are approximately equal in size or slightly larger than the microscopic pores in the microporous filament may be forced into the pores open at the outer peripheral surface of the filament or may be allowed to enter the passageway in the filament through the pores in the filament. This causes clogging of the microporous filament at the outer peripheral surface of the filament or results in reduction in the effective range of the sizes of the particles which can be collected by the hollow microporous filament serving as a filter element.
Another important drawback inherent in the prior-art device which disclosed in each of the patent specifications hereinbefore referred to, is that the hollow microporous filament of the device is held in position within the liquid conducting tube by the use of the joint element which connects the liquid conducting tube to another liquid conducting member or to the needle assembly included in the liquid dispensing device. Furthermore, provision of the joint element for the mounting of the filter element adds to the complexity of the overall configuration of the dripset and to the number of the steps to be taken by the practiser of the administration when in handling of the dripset.
Because, of the fact that the hollow microporous filament used in the prior-art medical liquid dispensing device is arranged to serve not only as a filter element but also as a partition element as previously mentioned, the filter element is capable of collecting solid particles only from the liquid entering the filter element from the passageway upstream of the filter element. For this reason, the hollow microporous filament serving as the filter element is permitted to collect solid particles only on its outer peripheral surface. As a consequence, the inner peripheral surface of the microporous filament is used merely as a surface to emit the filtered liquid into the passageway in the filament. Thus, the liquid medicament or the transfusion blood passed through the hollow microporous filament is filtered only once by the microporous filament. Thus the solid particles contained in the liquid medicament or blood have only one chance of being removed from the medicament or blood by the filter element of the microporous filament.
The present invention contemplates resolution of these problems inherent in prior-art liquid dispensing devices having filter elements, especially those using hollow microporous filaments.